Studies of IBr, ICl, and I2Cl6 Crystal Properties by Means of the Mössbauer Effect in 129I. II. Lattice Dynamics

Abstract

Lattice‐dynamics information on polycrystalline I2, IBr, ICl, and I2Cl6 molecular crystals was obtained by means of the Mössbauer effect in the 27.8‐keV line of 129I. In the case of I2 and IBr, which are isomorphous, evidence was found that the mean‐square displacement is larger in the direction of the molecular axis, and from considerations regarding the molecular arrangements, it is concluded that the larger 〈x 2 〉 is in the ac plane. Similar evidence was found also for ICl. No anisotropy in 〈x 2 〉 was detected in the case of I2Cl6. From the temperature dependence of the recoilless fraction and from its absolute value, the Debye temperature was calculated and found to be 58° ± 2°K for both I2 and IBr, suggesting a similar density of phonon states, in accordance with far‐infrared results obtained with the halogens at 77°K. The 〈x 2 〉 found for I2Cl6 is compared with that calculated from Raman data for ICl4 −, and it is concluded that the I‐Cl intramolecular vibrations are not excited by the recoilless mechanism, which explains the relatively large effect observed.